The present invention relates to a method for setting the engagement point of a friction clutch of a multi-step transmission for a motor vehicle, in particular a friction clutch of a double clutch transmission.
In drive trains with a spur gear transmission, a friction clutch, which has the function of a starting clutch and disconnecting clutch, is generally arranged between the spur gear transmission and a drive engine (such as an internal combustion engine). In double clutch transmissions, a double clutch arrangement is provided between the two component transmissions and the drive engine. These friction clutches can be embodied as dry-running or wet-running clutches and are subject to a certain degree of wear. In conventional manually-shifted transmissions, the increasing wear of the clutch is compensated by modified activation of the clutch on the part of the driver.
In automated spur gear transmissions (such as automated shift transmissions (AST) or double clutch transmissions (DCT)), the friction clutch is generally activated by an assigned shift clutch actuator. The clutch actuator can be, for example, a hydraulic actuator or an electromechanical actuator.
The actuator can additionally be embodied so as to be controlled either by pressure, force or travel.
The friction clutch is, as a load clutch, configured to transmit, at least briefly, even high torques which correspond to the torque which is output by the drive engine, or which are usually even larger. For comfortable engagement of the clutch, the friction clutch has to be changed from the opened state to the closed state via a slipping state. In this context, the input element and the output element of the friction clutch come to bear one against the other, with a torque being transmitted starting from a certain time. A point (a manipulated value of the clutch actuator) at which the friction clutch transmits a certain, relatively small torque, for example a value <20 Nm, is defined as an engagement point of the friction clutch at which a certain degree of transmission of torque is possible.
For safe, comfortable and also fast shift processes it is highly important in automated multi-step transmissions to know at which manipulated value of the clutch actuator this engagement point is reached. As mentioned above, this manipulated value can change owing to wear (for example of the clutch plates), mechanical tolerances or through other interference variables (for example setting of the clutch springs) in the course of time. Parameters which can change over a relatively short time, such as for example the temperature, also play a role here.
Various methods have become known for setting the engagement point of a friction clutch.
Document DE 196 52 244 A1 relates to a method for adapting the kiss point, wherein the engine torque is measured as an important parameter. A basic approach is to set a plurality of operating points in order to achieve reliable adaptation on the basis of said operating points by forming mean values.
WO 2004/076224 A1 discloses determining the torque which is transmitted by a friction clutch by comparing the rotational speeds of the engine and the transmission input, specifically while taking into account the synchronizing force, in particular in the overrun mode, while the engagement force is increasing.
A characteristic curve adaptation when the engine is running is known from WO 2004/076225 A1, wherein the clutch is closed with a defined value, subsequently a gearspeed is increasingly synchronized and then the synchronizing manipulated value at which the rotational speeds of the transmission input and the engine move away from one another is determined.
EP 0 931 961 A1 relates to a calibration method for a control clutch at a constant engine rotational speed, wherein the clutch calibration value is acquired from a target rotational speed change time which is determined from a profile resistance time. The profile resistance time represents a change in rotational speed which depends on the frictional characteristic, wherein, in particular, the clutch is opened and closed and the time within which the rotational speed of a gearwheel changes by a specific absolute value is measured.
A similar iterative method is known from EP 0 859 171 A1.
Document DE 195 40 921 A1 relates to the idea of randomly actuating a clutch in order to produce the relationship between the clutch torque and the manipulated variable.
A method for controlling the transmission of torque is also known from DE 199 39 818 C1. In this context, the velocity due to the engagement of a clutch is to be unequal to zero, wherein a parallel clutch is kept disengaged. The engagement point of the parallel clutch is determined by at least partial engagement of this clutch, wherein, in particular, the change in the rotational speed over the time of one half of a clutch is measured.
DE 102 44 393 A1 relates to the determination of the engagement point by measuring the pressure profile and by determining the first derivative thereof on a hydraulic cylinder.
DE 100 54 867 A1 discloses a method for determining a creepage point of a friction clutch. A first creepage point is set and the clutch torque is measured. A second creepage point is then set if there is no correspondence present in the first step. In this context, a value between the biting point and the first creepage point is then selected as a reference point. The new second creepage point is then set in such a way that a correspondence occurs between the desired torque and the torque which is actually transmitted.
A method for determining an engine friction torque is known from DE 101 13 700 A1. The method is used to determine, from the engine torque, the torque which is present at the friction clutch.
In addition, a method is known for determining a grip point of a friction clutch from EP 1 741 950 A1. A friction clutch is opened so that the transmission input rotational speed drops. The clutch is then closed again until the drop in the rotational speed is stopped. The clutch is subsequently closed further until the rotational speed remains the same or drops. The grip point is calculated from this.
A method for controlling a change of gearspeed in which the engine torque and the clutch torque are controlled is known from DE 101 01 597 A1. The control takes place as a function of differences in rotational speed.
Document DE 197 51 455 A1 relates to a method for controlling a clutch in which a set point clutch capacitance is set by means of a closed-loop controller on the basis of a characteristic function. The characteristic function is continuously adapted here.
A method for changing gearspeed is known from DE 102 24 064 A1. In said document, synchronization is triggered by determining a “switch-off point” which depends on a gradient of the rotational speed of the synchronized gearwheel.
Finally, EP 1 067 008 A1 discloses a method for clutch characteristic curve adaptation, specifically for a double clutch transmission.
In this context, in each case the clutch characteristic curve of a clutch which is currently not being used to transmit a torque (referred to as the free friction clutch) is to be adapted. Said clutch is initially closed with a certain clutch actuating force, and the system waits until the transmission input shaft has reached the synchronizing rotational speed (that is to say is running at the same speed as the engine shaft). Subsequently, synchronization of the assigned free component transmission is activated until a sufficient difference in rotational speed is present. This synchronization is then released and the rotational speed gradient of the transmission input shaft is subsequently determined. The value of the previously transmitted clutch torque is calculated therefrom. On the basis of this value, a clutch characteristic curve adaptation is carried out in connection with the previously defined clutch actuating force.
In addition, a rapid method for setting the engagement point of a friction clutch is known from WO 2009/065591.